"Your SMTP (outgoing mail server) could be blocked. Here is how to test it:
1. Go to 'Start' -> 'Run' -> type: cmd
2. When the Command prompt appears type: telnet mail.supremecenter.com 25
3. Wait a few seconds. You must see the following: '220 mail.supremecenter.com ESMTP'. This will be OK.
4. If you do not see this message, your SMTP is being blocked. In this case try the following:
- If you are behind a firewall, we may be unable to authenticate you. If you have a personal firewall, try lowering the security.
- Call your ISP and ask if they block people sending through remote SMTP's. Some are doing that, though relatively few.
- You can use your ISP's SMTP server. In your email client, put the SMTP server that you use to send out your ISP email, and your email will still appear with your domain name. Also see the error messages explanations."
Extracted from: click here
Hope that is of some help?